The present invention is usable with a photoconversion element of the type in which charge carriers generated in response to incident light are stored in a control region of a transistor, and a signal related to the quantity of stored carriers is read out through a main electrode region. An element of this type, which is capable of providing an output signal in which the stored charge is amplified, is known from EP-A-0132076. The photoconversion element of that document is a bipolar transistor, and the carriers are stored in the base. The signal may be read out through the collector or the emitter. The base potential may be controlled via a capacitively coupled electrode, but the present invention is not limited to this. Also, the present invention is not limited to bipolar devices. EP-A-0132076 discloses an operation cycle for its photoconversion element in which incident light causes electrical carriers to be accumulated in the control region of the element during an accumulation period, a signal dependent on the amount of accumulated charge is read out through a main electrode region during a read period, and then the state of the control region is reset to a standard condition during a reset or refresh period.
EP-A-0222624 and EP-A-0253678 disclose a preferred arrangement in which two reset or refresh operations are carried out in succession. The first is a so-called complete refresh, in which charge carriers are injected into the control region at least in the condition that it has been exposed to very little light during the accumulation period so that the quantity of stored charge is very low, or alternatively the control region potential is set to a defined level, in either case in order to ensure that the control region has a greater quantity of stored charges in it than it has at the end of an accumulation period in which no light has been incident on the photoconversion element. This is followed by a so-called transient refresh operation in which the photoconversion element is biased so that charges stored in the control region flow out through a main electrode region.
This two-step refresh process has been found to be advantageous compared with using a transient refresh process alone. If the transient refresh condition is maintained for long enough, substantially all of the stored charges in the control region can be removed. However, if the transient refresh condition is maintained only for a relatively short time, enabling a faster operation of the photoconversion device, the level of stored charge in the control region is brought down to some particular non-zero standard level, provided that the level of stored charge was sufficiently above the standard level before the transient refresh operation began. However, where the stored charges are already at the standard level before the transient refresh operation begins, e.g. because the photoconversion element has not received any light during the accumulation period, the level of stored charges decreases slightly during the transient refresh operation so as to finish at a level below the standard level. Thus the level of stored charge in a photoconversion element which is not receiving light will slowly decrease during successive exposure periods, and when it next receives light its output signal will not be correct. By conducting a complete refresh operation before the transient refresh operation, it can be ensured that the control region contains sufficient stored charge at the beginning of the transient refresh operation to bring the level of stored-charge to the standard level at the end of the transient refresh operation.
In the arrangements disclosed in the prior art documents discussed above, where a photoconversion device comprises a two-dimensional array of photoconversion elements, the control circuitry typically refreshes the elements of a row simultaneously, and refreshes different rows at different times.
An arrangement is known from EP-A-0274236 in which the photoconversion element is a bipolar transistor and the emitter is held at ground level during a transient refresh operation. During a subsequent accumulation period the emitter is held at a standard voltage which is lower than the collector voltage, so as to limit the voltage to which the base can rise as it accumulates carriers, as a measure against "blooming" following an overflow of carriers from an element receiving very strong light.
EP-A-0132076, EP-A-0222624, EP-A-0253678 and EP-A-0274236 are incorporated herein by reference.
An example of a photoconversion device having a two-dimensional array of photoconversion elements is illustrated in FIG. 1. In this Figure, the photoconversion elements are bipolar transistors in which the base is controlled by a capacitively coupled electrode. However, as mentioned above, the present invention is not limited to this type of photoconversion element, and alternative photoconversion elements, including elements without a capacitively coupled control electrode for the control region and elements which are not bipolar transistors, are disclosed in EP-A-0253678.
In FIG. 1, a unit picture cell of the array comprises a photoconversion element or sensor (which is a bipolar transistor type device as disclosed in EP-A-0132076) T. The base of the sensor T is capacitively coupled to a control electrode, and this coupling is represented by a capacitor C. Additionally, the base is coupled to a P-channel FET (e.g. a MOSFET) M. Each row of the array of sensors has a common horizontal drive line 2, connected to the capacitor electrode and the gate of the FET M of each cell. Each column of the array has a common vertical output line 3, connected to the emitter of each sensor T. Each vertical output line 3 is connected to ground through a respective FET 4, for resetting the vertical output line 3. Each vertical output line 3 is also connected to a respective capacitor 5 for storing signals output from a sensor T onto the output line 3, through a respective FET 6. Each storage capacitor 5 is connected to a common horizontal output line 7 through a respective FET 8, and the common horizontal output line 7 provides an input to an amplifier 9. The common horizontal output line 7 is also connected to ground through an FET 10.
For controlling the operation of the device, each horizontal drive line 2 is connected through a respective FET 11 to a terminal 12 for supplying drive pulses, and the rows of the array are selected by outputs from a row shift register which turn on the FETs 11 in turn. Control pulses to a terminal 13 turn on the FETs 4 to ground the vertical output lines 3, and control pulses to a terminal 14 to turn on the FETs 6 to connect the vertical output lines 3 to the storage capacitors 5. Outputs from a column shift register selectively turn on the FETS 8 to connect the selected storage capacitors in turn to the common horizontal output line 7. Control pulses to a terminal 15 turn on the FET 10 to ground the horizontal common output line 7 to reset it. The photoconversion output signal from the device is provided from the amplifier 9 to an output terminal 16.
As can be seen in FIG. 1, an additional FET M is provided at one end of each row, so that an FET M is present at each end of each row. Except at each end of each row of the array of sensors T, the two main electrode regions (source and drain) of the FET M of each cell are connected respectively to the base of the sensor T of the same cell and the base of the sensor T of an adjacent cell. At each end of each row, the FET connects the base of a sensor T to a line leading to ground. Thus, when a horizontal drive line 2 turns on the FETs of one row of sensors, the bases of all the sensors in the row are connected through the FETs M to ground. The additional FET M in each row can be omitted, in which case the line of FETs is connected to ground at one end only.
FIGS. 2A-2C illustrate waveforms of the signals applied to the terminals 12, 13 and 14. Throughout this specification the convention will be used that the waveform applied to a terminal will be indicated by .phi. followed by the number of the terminal.
During the accumulation period, when the photoconversion device is exposed and charges are accumulated in the base of each sensor T in accordance with the amount of light incident on the sensor, the relevant row of the array of sensors is not selected by the row shift register. If none of the rows is being read or refreshed .phi.12 is at ground, .phi.13 is high to connect the vertical output lines to ground, and .phi.14 is low to isolate the vertical output lines from the storage capacitors 5. When a row is selected for reading and refreshing, the row shift register turns on the corresponding FET 11.
For the reading operation .phi.13 goes low to isolate the emitters of the sensors T and the vertical output lines 3 from ground, and .phi.14 goes high to connect the emitters and the-vertical output lines 3 to the storage capacitors 5. .phi.12 goes high sufficiently to raise the potential of the bases of the sensors T in the selected row through the action of the capacitor C to turn on the transistor type sensors T so that an output signal corresponding to the quantity of charges stored in the base region is provided through the emitter region to the respective vertical output line 3 and is stored on the respective storage capacitor 5. The positive pulse of .phi.12 firmly turns off the FETs M, so that the base voltage level is not influenced by the ground connection through the FETs M during read-out. Subsequently, the column shift register turns on the FETs 8 in turn to read out the signals from the storage capacitors 5 to the common horizontal output line 7 and the amplifier 9.
For a refreshing operation, .phi.13 goes high and .phi.14 goes low to isolate the emitters and the vertical output lines 3 from the storage capacitors 5 and connect them again to ground, and .phi.12 goes sufficiently low to turn on the P-type FET M. The low-going pulse of .phi.12 will initially bias the base of each sensor to turn if off, and then the base of each sensor will become grounded through the FETs M, with charges flowing into or flowing out of the base region depending on the quantity of charge carriers which have been accumulated during the accumulation period. This provides the so-called complete refresh operation, in which the base of each sensor T of the selected row is set to a common potential.
Next, .phi.12 goes high once again to turn off the P-type FETs M and forward bias the base of the sensor T relative to the emitter by the action of the capacitor C. The emitter remains connected to ground through the FET 4. This provides the so-called transient reset, during which charges stored in the base region are removed through the emitter region. Because the base of each sensor T of the selected row has been set to a standard level through the FETs M during the complete reset operation, all of the base regions of the sensors in the row are set to a common refresh level at the end of the transient refresh operation. .phi.12 then returns to the ground level to begin the next accumulation period, and the row shift register can turn off the relevant FET 11 to de-select the row.
As will be appreciated by those skilled in the art, it is possible to provide an alternative construction in which a first set of horizontal drive lines controlled by a first row shift register are used exclusively for read out, and a second set of horizontal drive lines, controlled by a second row shift register, are used exclusively for refreshing, and such an arrangement is illustrated schematically in FIG. 3.
In FIG. 3, read out is controlled by a read out row shift register 30 with a timing tR and refresh is controlled by a refresh row shift register 31 with a timing tC. By controlling the period between the drive timing tR of the read out row shift register and the drive timing tC of the refresh row shift register, the accumulation period for each sensor can be controlled, providing the so-called electronic shutter function. In this Figure, a column shift register 32, the photoconversion array 33 and the output amplifier 34 are also illustrated schematically.
The arrangement of FIGS. 1 and 2, 2A-2C has the advantage that, by providing a common control line for the capacitor electrode controlling the base through capacitor C and the gate of the FET M, the FET M is automatically driven reliably into the off state during read out and the transient refresh operation. However, the total safe voltage range between the highest voltage and the lowest voltage which may be applied to the horizontal drive lines 2, is defined by the characteristics of the components of the photoconversion device, and particularly by their maximum withstand voltages. Consequently, the fact that the horizontal drive lines 2 must go negative in order for the complete refresh operation to be carried out limits the maximum positive voltage which can be applied during transient refresh and read out. This limits the extent to which the transistor type sensor element T can be turned on, so that the read out signal remains relatively low and the maximum output signal when the sensor is saturated by bright light is relatively small. Additionally, the circuit for supplying the drive waveform .phi.12 must be relatively complicated as three different levels are required for this waveform.
In the devices of FIGS. 1 to 3 a problem can arise when a still picture is being taken, for example if the device is used in a still video camera, particularly when a still picture is required of a moving object. Because each row is read and refreshed at a different time, the accumulation periods for different rows are provided with slightly different timings. This results in an image distortion. This image distortion can be eliminated by providing a mechanical shutter which is opened after all of the rows have been refreshed and is closed before any of the rows are read. However, in this case there may still be an undesirable dark current during the time taken to read and refresh all of the picture cells.
The present invention in its various aspects seeks to solve or reduce the various problems discussed above or provide improvements over the arrangements described.